This invention relates generally to reference apparatus for navigable vehicles such as aircraft, and more particularly, to rate/acceleration sensors used in an attitude and heading reference system. Advancement in the art of precision flight control and guidance apparatus for aircraft, missiles, and space vehicles depends in part on progress in sensor technology. Present computer technology allows sophisticated and complex signal processing at reasonable cost, but the information processed is frequently derived from sensors having a cost which is a disproportionate part of the system cost.
A reference system having inertial instruments rigidly fixed along a vehicle-based orientation reference wherein the instruments are subjected to vehicle rotations and the instrument outputs are stabilized computationally instead of mechanically is termed a gimballess or strapped-down system. Such systems generally include computing means receiving navaid data such as magnetic and radio heading; air data such as barometric pressure, density, and air speed; along with output signals of the inertial instruments for generating signals representative of vehicle position and orientation relative to a system of coordinate axes, usually earth oriented. The presence of high angular rates associated with strapped-down systems adversely affects performance and mechanization requirements. Consequently, such reference systems have been used extensively in missiles, space, and military vehicles, but their use in commercial aircraft has been less extensive because of economic constraints associated with the manufacture of precision mechanical assemblies, i.e., gyroscopes and other precision sensors.
Strapped-down inertial reference systems become practical for commercial aircraft from the standpoint of cost of ownership, weight, reliability, and maintainability with the advent of small, lightweight, highly accurate and relatively low-cost rate sensors and accelerometers. Angular rate sensing apparatus utilizing rotating piezoelectric generators are known; see for example U.S. Pat. Nos. 2,716,893 and 4,197,737. Such devices generally comprise piezoelectric generator elements mounted to a rotatable drive shaft and oriented for generating signals responsive to particular bending forces sensed by the instrument. The processing of signals derived from such instrumentation involves the measurement, amplification and transmission of very low level DC and low frequency signals.
While many of the problems inherent in the prior art have been overcome by the systems and techniques set forth in the aforementioned co-pending applications, all of which are herein incorporated by reference in their entirety, there are still problems which must be overcome to improve the accuracy of the systems and reduce the cost and complexity of construction. In particular, in one of the above-mentioned applications, two transducers are used to sense linear acceleration. The transducers are placed 90.degree. apart in rotation in order to cancel known bias problems in the two-axis accelerometer output due to twice-spin frequency vibrations. The vibrations are present due to imperfections in the motor in particular, as well as possible external sources. In that system and technique, the undesired bias terms were cancelled by phase-shifting the signal from the leading transducer by 90.degree. and adding it to the signal from the first transducer. Upon demodulation, the output was provided to a two-pole lowpass filter to limit the ripple generated from demodulating and phase detecting the sinusoidal accelerometer signals. This lowpass filter is the primary limitation on the bandwidth of the system and thus prevents use in environments requiring large bandwidths.
Furthermore, the structure used to measure alternating accelerations in the two-axis systems proposed above produce cross-axis coupling problems. For low frequency and static accelerations, such cross-axis coupling is negligible. In other cases, however, the cross-axis coupling is significant and is caused by the lowpass filter used to phase shift one of the transducer signals in the aforementioned systems. For broadband applications, the cross-axis coupling is undesirable and must therefore be reduced in order to provide less restrictive operating parameters so that the assemblies may be used for greater applications.
The rotating structure in the above-mentioned co-pending applications also produced improved results, but there is a further need for structures which enable miniaturization and configurations which enable demodulation to widen the bandwidth. Since the cost of producing the assembly is directly related to the complexity of the parts and their interrelationship, a reduction in the number of parts and the size of the assembly can significantly improve the applications in which the sensor may be employed. In addition, there is a need to provide sensors which may be easily disassembled for repair so that the cost of construction and replacement can be reduced.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide an improved transducer assembly and demodulating system for providing two-axis rate and acceleration measurements.